What you are seeing in this picture is a magnified image of the rear legs of a common planthopper insect. And those teeth you see, called trochanter bumps, are the only-known example of gears that evolved in nature.

The trochanter is the segment of the planthopper that attaches to its hips. Nobody had really known why they were there until two British scientists were trying to figure out how planthoppers could coordinate their legs so perfectly while jumping. Turns out, those trocanter bumps were acting as gears — the tiny hopper pushes its trochanters together before jumping and thus keeps the motion of the two legs in sync.

Ed Yong has pretty hilarious article about this today on Not Exactly Rocket Science. He writes:

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Gears allow two machines to rotate together in opposite directions. That’s exactly what the planthopper’s trochanter bumps do. [Mechanical engineer Gregory] Sutton tested this by pulling on the tendons of its jumping muscles with some forceps (“It’s the Serious Edition of Operation”, he says.). Even if he only pulled one tendon, both legs would extend because the gears transmitted the motion of one trochanter into the other.

“Then, we got really lucky because we saw a few jumps where the gears wouldn’t engage perfectly,” says Sutton. When this happened, one leg was partially extended before the gears finally snagged and the planthopper’s nigh-perfect coordination was ruined.

Wait! It gets better. These gears are training wheels!

The planthopper nymphs lose them when they become adults. But the adults don’t shoot off in uncoordinated spins—if anything, they’re better jumpers than the youngsters. Their hind trochanters make much closer contact with each other, and Sutton thinks that the friction between them helps to keep them in time. “We’re kind of sure about that, but not entirely sure,” he says.

“This is to our knowledge the first time that proper, engaging, counter-rotating gears have been seen in the animal kingdom,” says Sutton. Crocodiles have cog-like teeth in their heart valves, and the wheel bug and cog-wheel turtle have teeth on their shells. But none of these structures actually act like gears. “You never see one cog-wheel turtle saddle up next to another, engage their shells, and spin in opposite directions,” says Sutton. “If you did, I want you to call me. If I see that on your website, and I haven’t been called, I will be an angry man.”